In x-ray transmission imaging, illumination is projected through an object and the imaging signal results from a subtractive process, i.e. what is imaged is the far field of the illumination minus any signal that was absorbed, reflected or scattered. In many industries x-ray inspection is being used routinely for inspection of products in the manufacturing environments. Many of these applications require that a particular material be examined in the inspection process in spite of the presence of other materials that also absorb x-rays. For example, surface mounted integrated circuits are examined during manufacture to determine the distribution of solder on the assembly, which in turn, is related to the reliability of the assembly. In such systems, Cu traces, Si chips, and Fe in transformers all conspire to obscure, or ‘shadow’ the solder joints.
This shortcoming of projection imaging is overcome by CT (computed tomography) scanners. CT scanners combine information from a variety of projection viewpoints to overcome the shadowing and generate a 3D (three-dimensional) description of the object. A typical CT system measures the x-ray flux reaching a detector from a source that moves around the object. The object being scanned is modeled by a plurality of voxels having unknown x-ray absorbency. At each point, the measured flux represents the weighted sum of the x-ray absorbencies of each voxel along the path from the x-ray source to the detector. Different paths provide weighted sums involving different sets of voxels. If sufficient points are measured, a data processing system can solve the resulting system of equations for the x-ray absorbency of each voxel. The resulting data can then be analyzed or displayed as a three-dimensional model of the object that can be viewed from different viewpoints.
CT scanners are widely used for imaging the human body as part of diagnostic procedures. In principle and in limited practice, such scanners would be useful in imaging inanimate objects on assembly lines. Unfortunately, the cost of this equipment and its relatively low throughput has inhibited the use of CT scanners for such high volume applications.
One specialized form of tomographic scanning, known as laminography, is utilized in the inspection of printed circuit boards and other thin objects in which the image of a particular horizontal “slice” of the object is needed. In the case of a printed circuit board in which the solder layer is being inspected, only the slice on which the solder layer is placed is of interest. In laminography systems, a number of projection images of the object being examined are averaged together to form an image of one plane through the object. The images are chosen such that the plane of interest will generate the same image in each of the views while the planes above and below the plane of interest produce images that vary from image to image. When the images are summed, the images from the planes that are out of focus generate a diffuse background while the images from the in focus plane add constructively. To provide the averaging of the images, two of the three components of the system, the object, the x-ray source, and the detector, must move relative to the third. The earliest systems utilized a mechanical system in which an x-ray source and an imaging detector are rotated about an axis that is perpendicular to the focus plane. The imaging detector is generally a bulk scintillator with an optical system that images the image generated by the scintillator onto a CCD camera. The efficiency of this type of imaging system is a few percent. As a result, this type of system has a poor signal-to-noise ratio. To overcome this poor signal-to-noise ratio, the number of images that must be averaged must be increased, thereby decreasing the throughput of the inspection system. In addition, the cost associated with the mechanical system significantly increases the cost of the system.
Systems in which the motion of the two components is executed electrically are also known. For example, U.S. Pat. No. 6,324,249 describes a system in which a scanning x-ray source is combined with a very large x-ray detector consisting of a scintillator and a CCD imaging array. The x-ray source generates a point x-ray source that moves relative to the object. The scintillation detector is placed under the object and connected to the CCD array either by light pipes or a lens system that images the light output of the scintillator onto the CCD array. The CCD is configured as a two-dimensional array of detectors in which each column of detectors is readout by shifting the measured values from one-detector to the next in the column until the values leave one end of the column.
While such systems avoid the cost and time constraints imposed by systems that utilize mechanical motion, these systems are still too slow for many industrial applications. The scintillation detector is far from the CCD detector in embodiments that utilize a lens to form an image on the CCD array. As a result, the efficiency of the detector is relatively small, which leads to long integration times. Second, the CCD arrays are arranged such that the entire array must be readout for each x-ray image position, even though a small fraction of the array is needed for each x-ray position. As the array size is increased to accommodate larger printed circuit boards, the readout time becomes a significant fraction of the measurement time.